Method of storing petroleum oil



whereby moisture absorption in atmospheric conditions,

- and pressure.

3,098,702 I METHOD OF STORING PETROLEUM OIL Alfred L. Baker, .lr., Brinton Lea, Thornton, Pa., assignor to Sun Oil Company, Philadelphia, Pa., a corporation of New Jersey N Drawing. Filed Oct. 26, 1960, Ser. No. 64,989 1 Claim. (Cl. 21-605) This invention relates to the bulk storage of petroleum oil. It particularly relates to the storage of electrical oils whereby an increase in water content of the oil is prevented. It especially relates to an improved method of storing electrical oils in a storage vessel equipped with breathing means.

Particular non-volatile petroleum oils, that is, oils having a Saybolt viscosity above about 50 seconds at 100 F. and which boil mainly above 400 F., such as turbine oils, white oils, spindle oils, transformer oils and the like must be essentially free from dispersed water if their quality is to be deemed satisfactory. It has been found that sus-' pended water imparts a cloudy appearance which reduces marketability of the oil. More important, however, is the fact that oils which are used for dielectric purposes must be essentially free from dissolved water as well as suspended water. Otherwise, the dielectric strength and other electrical properties of the oil are adversely affected. For these reasons, the transformer oils are treated under essentially anhydrous conditions and are sent to bulk storage containing no dispersed water and containing from less than 30 parts per million (p.p.m.) of dissolved moisture.

A transformer oil containing less than 30 p.p.m. dissolved moisture, and preferably less than 20 p.p.m., will have a satisfactory dielectric value. However, it has been found that an oil, upon storage in a vessel which has breathing means, will absorb moisture from the ambient air that is breathed into the tank. It is not uncommon for oil under such storage conditions to increase in water a content 12-20 p.p.m. in one week or even 20-75 ppm. in two weeks. When, for example, transformer oil contains 30-75 ppm. of moisture or more it is unsatisfactory for a dielectric uses. Excessive moisture in the oil shortens the life of transformer insulation, causes sludging of the oil, produces electrical losses and increases operating temperatures of the transformer.

It is an object of the present invention to provide an improved method for storing non-volatile petroleum oils is minimized.

The present invention applies to the storage of nonvolatile petroleum oils in bulk storage vessels which normally would have breathing means.

The term breathing means as used herein is defined as any means whereby internal tank pressure is equilibrated to external changes i.e., changes in temperature Normally, a non-volatile hydrocarbon storage vessel contains valve mechanism whereby ambient air is passed into the vapor space of the tank when the external air is warmer than the internal air. On the other hand, the vapor space of the tank is partially exhausted as the external air temperature cools below the internal equilibrium temperature. In this manner, a storage vessel breathes. The term also includes those storage vessels having simply an open vent.

In accordance with the present invention, moisture absorption by stored petroleum oils is prevented by passing a stream of compresesd, dried air over the surface of the oil in storage.

Thus, the present invention is based on the discovery 7 that ambient air compressed to between 30 and 500 pounds per square inch gauge (p.s.i.g.), cooled to ambient temperature (e.g. 35 to 80 F.), and freed of condensed moisture will have a dew point of less than 50 F., usually be- Patented July 23,1963

low 35 F., and is sufiiciently dry to retain the water content of petroleum oil within the range of 10 to 20 ppm. during normal periods of storage. The air can be introduced continuosly or periodically. Usually, it is preferred to circulate the air continuously over the surface of the stationary oil under a forced draft. The minimum air flow rate is defined as that which will maintain a slight positive pressure on the outlet vent.

The invention, while operable to the storage of some volatile hydrocarbons, should be restricted to non-volatile oils. Treatment of lower boiling fractions by the invention might cause excessive hydrocarbon losses, through vaporization, and might cause hazardous operating conditions.

The following examples illustrate the invention:

Example I Transformer oil having a Saybolt viscosity of 56 seconds at 100 F., boiling between 500 and 700 F., and containing 17 p.p.m. moisture was pumped into a 2500 barrel capacity storage tank which was equipped with standard breathing valves. After two weeks of storage, the water content of the oil had increased to 31 p.p.m.

The above example illustrates the conventional method of storing oils. It is clear that this method is unsatisfactory for storing transformer oils since an oil containing 31 ppm. of moisture is undesirable for dielectric use.

Example 11 Example III The transformer oil from Example I was dumped into another 2500 barrel capacity storage tank which was equipped with a compressed air hose connected to one side of the tank. On the opposite side, an outlet vent was available. For two weeks, air was compressed to p.s.i.g., cooled to 70 'F., freed of condensed water by passage through a knockout pot, and introduced into the vapor space above the surface of the oil. It was passed under forced draft over the surface of the oil and removed under a slight positive pressure from the vent on the opposite side of the tank. The stored oil, at the end of the two week period, tested 17 ppm. water, which was the same as its original water content. The air entering and leaving the tank had a dew point between 30 and 35 F.

Thus, as shown in Example III the present invention provides a simple, economical method of storing petroleum oils having a susceptibility for moisture whereby the absorption of moisture is prevented. The invention is a combination method which comprises (1) confining the petroleum oil as a stationary liquid in a storage vessel equipped with breathing means; (2) compressing ambient air to a pressure between 30 and 500 p.s.i.g., cooling to ambient temperature, and removing the condensed water therefrom; (3) introducing the treated air into the vapor space above the surface of the oil; (4) passing the treated air over said surface; and (5) removing the air from the storage vessel at a level above the surface of the oil.

-I claim:

The method of storing non-volatile petroleum oil containing less than 30 parts per million water which comprises in combination: confining the petroleum oil as a stationary liquid in a storage vessel equipped with breathing means; introducing a stream of hereinafter specified air into said vessel at a level above the surface of the oil; compressing ambient air to a pressure Within the range of 30 to 500 pounds per square inch gauge, cooling said compressed air until a dew point of less than 50 F. is attained, removing from said cooled air the condensed water, and introducing the treated air into said vessel as hereinabove specified; passing said treated air over the surface of the oil, and removing from said vessel at a level above the surface of the stored oil the stream of air having substantially unchanged Water content.

References Cited in the file of this patent Perry, Chemical Engineers Handbook UNITED STATES PATENTS OTHER REFERENCES 3rd ed., 1950, I 

